Re: objtool warning "uses BP as a scratch register" with clang-9
From: Geert Uytterhoeven
Date: Wed Sep 04 2019 - 07:54:09 EST
On Thu, Aug 29, 2019 at 8:31 PM Linus Torvalds
> On Thu, Aug 29, 2019 at 10:35 AM Josh Poimboeuf <jpoimboe@xxxxxxxxxx> wrote:
> > Peter suggested to try WRITE_ONCE for the two zero writes to see if that
> > "fixes" it.
> I'm sure it "fixes" it.
> .. and then where else will we hit this?
> It's one thing to turn a structure zeroing into "memset()", but some
> places really can't do it.
> We use "-ffreestanding" in some places to make sure that gcc doesn't
> start calling random libc routines. I wonder if we need to make it a
> general rule that it's done unconditionally.
> Sadly, I think that ends up also disabling things like
> "__builtin_memcpy()" and friends. Which we _do_ want to have access
> to, because then gcc can inline the memcpy() when we _do_ use
> We used to do all of those heuristics by hand, but wanted to let the
> compiler do them for us.
> - we do want "memcpy()" to become "__builtin_memcpy()" which can then
> be optimized to either individual inlined assignments _or_ to an
> out-of-line call to memcpy().
You can do
#define memcpy(d, s, n) __builtin_memcpy(d, s, n)
Alpha, m68k, sparc, and x86 already do.
Geert Uytterhoeven -- There's lots of Linux beyond ia32 -- geert@xxxxxxxxxxxxxx
In personal conversations with technical people, I call myself a hacker. But
when I'm talking to journalists I just say "programmer" or something like that.
-- Linus Torvalds